In prior art it is known that when a quarter wave optical retardation plate or film (QWF) is used in combination with a linear polarizer, the linearly polarized light transmitted by the polarizer will be transformed into circularly polarized light. Alternatively, if a QWF is used in combination with a circular polarizer, the circularly polarized light transmitted or reflected by the polarizer will be transformed into linearly polarized light.
QWFs are particularly useful for combination with a broadband cholesteric polarizer reflecting circular polarized light of a broad wavelength range and a single handedness and transmit circular polarized light of the same wavelength band and opposite handedness.
As QWF usually a stretched polymer film of e.g. polyvinylalcohol (PVA) or polycarbonate (PC) is used.
A combination of a QWF and a broadband cholesteric polarizer is disclosed for example in EP 0 606 940, WO 96/02016, WO 97/35219 and EP 0 982 605. For example EP 0 606 940 and WO 96/02016 disclose a linear polarizer consisting of a broadband cholesteric polarizer and a QWF comprising a stretched film of isotropic polymer. The circular polarized light transmitted by the broadband cholesteric polarizer is transformed into linear polarized light by the QWF. The combination of QWF and broadband polarizer is suitable as replacement of standard absorption polarizers and/or as light enhancement film for displays as described in these documents.
However, prior art QWFs comprising stretched polymer films as described above do exhibit only moderate optical anisotropy. Therefore, to achieve the desired net retardation, which is the product of birefringence and film thickness, a high film thickness is required, which is disadvantageous especially for use in flat panel displays. Furthermore, when a liquid crystal display comprising a cholesteric polarizer and a QWF as described e.g. in EP 0 606 940 or WO 96/02016 is watched under an increasing viewing angle, its optical properties like the luminance and the contrast ratio are deteriorating.
Therefore one aim of the present invention is to provide an optical QWF retarder for use in combination with a linear or circular polarizer, in particular for use in combination with a broadband reflective polarizer, that does not have the drawbacks of prior art films.
WO 98/04651 discloses a combination of a broadband cholesteric polarizer with a QWF retarder comprising polymerized liquid crystal (LC) material with planar orientation. WO 98/12584 discloses a QWF retarder comprising a polymerized LC material with tilted orientation. These films give improved properties and allow lower film thickness compared to stretched plastic films. However, they are disadvantageous for mass production due to the fixed orientation of their optical axis.
Thus, for mass production roll-to-roll lamination of the QWF to other optical films, like compensators or polarizers, is desired. However, some applications require that the optical axes of the QWF and the adjacent optical film are not parallel but twisted at an angle to each other. In this case, roll-to-roll coating or lamination of the films is not possible, but instead piece-to-piece lamination of the QWF to the adjacent optical film is necessary. This is more complicated and time-and material-consuming especially when the optical film pieces have to be cut at an angle, e.g. in case the optical axes of adjacent films are aligned neither parallel nor at right angles, but at oblique angles within the film plane. In case of optical films comprising polymerized LC material, alternatively special rubbing techniques must be applied to achieve alignment of the optical axis at oblique angles in the film plane.
Therefore another aim of the present invention is to provide an optical retarder suitable for use in combination with other optical films like polarizers and/or compensators, which is easy to manufacture and is particularly suitable for mass production.
The inventors have found that this aim can be achieved by providing a retardation film comprising a mesogenic or LC material with twisted structure and planar orientation, for example a cholesteric LC material. In the following such a film will be shortly referred to as twisted A-plate.
In the twisted A-plate film according to the invention the mesogens or LC molecules of the mesogenic or LC material are oriented substantially parallel to the plane of the film and with their long molecular axis oriented in a preferred direction within molecular sublayers, said preferred orientation direction in different molecular sublayers being twisted at an angle φ around a helix axis. In case of an optically uniaxial LC material with positive birefringence the preferred orientation direction of the LC molecules corresponds to the optical axis in each sublayer. Thus, a twisted A-plate according to the present invention includes multiple molecular sublayers with different directions of the optical axis.
The twisted A-plate according to the present invention has the manufacturing advantage that the direction of alignment can be set at any angle and the retardation determined by the twist and the thickness of the film. This will allow roll-to-toll coating rather than piece-to-piece lamination with other optical films, where non-parallel alignment of the optical axes is required.
Optical films with twisted structure are known in prior art and have been suggested as compensators for TN and STN displays, for example in EP 0 423 881, EP 0 576 931 and U.S. Pat. No. 5,243,451. The optical films described in these documents comprise a twisted nematic LC material, e.g. a nematic polymer, with a twist angle in the same range as that of the LC medium in the display cell that is to be compensated, i.e. between about 90° for TN cells and up to 270° for STN cells. GB 2,315,072 discloses an optical film comprising a polymerized cholesteric LC material with helically twisted orientation, wherein the helical pitch is selected such that the film shows reflection of circular polarized UV light, and its use as a circular UV polarizer. However, there is no suggestion in the above documents to use twisted retardation films as OWF in combination with a circular or linear polarizer.